Rapid variability of 3C273 at 300 GHz

With a broadband bolometer mounted on the 3.6 m ESO telescope the authors have measured 3C273 at 300 GHz 6 times in 6 days. During this series of measurements they observed a sudden drop in flux density by 11.6 +- 2.9 Jy (spectral index assumed to be zero, and all known errors included). In an interpretation with the model of bulk relativistic motion the brightness temperature inferred can be combined with the observed superluminal motion, and strict limits on the model parameters can be derived. (Auth.)